1. Field of the Invention
The present invention relates to photoelectric conversion apparatus of the type which stores carriers produced by optical excitation.
2. Related Background Art
FIG. 1A is a schematic cross section view of an example of photoelectric conversion cell disclosed in U.S. Pat. No. 4,686,554 (EP 0132076). FIG. 1B is an equivalent circuit diagram of the photoelectric conversion cell.
In FIGS. 1A and 1B, a plurality of photoelectric conversion cells are formed and arranged on an n-silicon substrate 1 and separated electrically from adjacent cells by element separating areas 2 made of SiO.sub.2, Si.sub.3 N.sub.4 or polysilicon.
Each cell has the following structure. A p-area 4 is formed by doping p-type impurities into an n.sup.- -area 3 of low impurity density formed using epitaxial techniques or the like An N.sup.+ -area 5 is formed in the p-area 4 by impurity diffusion or ion implantation. The p-area 4 and n.sup.+ -area 5 are the base and emitter, respectively, of a bipolar transistor.
An oxide film 6 is formed as an insulating layer on the n.sup.- -area 3 on which those areas are formed and a capacitor electrode 7 with a predetermined area is formed on the oxide film 6. The capacitor electrode 7 opposes the p-base area 4 having the oxide film 6 therebetween and the potential of the floated p-base area 4 is controlled by a pulse voltage applied to the capacitor electrode 7.
Furthermore, an emitter electrode 8 connected to the n.sup.+ -emitter area 5, an n.sup.+ -area 11 of high impurity density provided on the back of the substrate 1, and a collector electrode 12 through which a potential is applied to the collector of the bipolar transistor are formed.
The basic operation of the cell as shown in FIGS. 1A and 1B will now be described. First, assume that the p-base area 4 of the bipolar transistor is at an initial negative potential. Light 13 enters the cell from the side of the p-base area 4, positive holes of electron-positive hole pairs generated by the incident light are stored in the p-base area 4, and increases the potential of the p-base area 4 in the positive direction (storage operation).
Subsequently, a positive read voltage pulse is applied to the capacitor electrode 7 to thereby cause the floating emitter electrode 8 to output a read signal corresponding to a change in the base potential in the storage operation (reading operation). Since the quantity of electrical charges stored in the p-base areas 4 does not substantially decrease, repetition of the reading operation is possible.
In order to eliminate positive holes stored in the p-base area 4, the emitter electrode 8 is grounded and a positive refresh pulse voltage is applied to the capacitor electrode 7. By application of this pulse, the p-area 4 is biased forwardly relative to the n.sup.+ -emitter area 5 to thereby eliminate the positive holes stored therein. When the refresh pulse falls, the p-base area 4 is restored to its initial state (refresh operation). Subsequently, store, read and refresh operations are repeated in a similar manner.
In summary, according to the above described photoelectric conversion method, the carriers produced by the incident light are stored in the p-base area 4 and the quantity of electric charges of the carriers controls the electric current flowing between the emitter electrode 8 and collector electrode 12. Thus, the stored carriers are amplified by the amplifying function of each cell and read to thereby achieve high output, high sensitivity and low noise.
The potential V.sub.p produced at the base due to the carriers (here, holes) stored therein by optical excitation are given by Q/C where Q is the quantity of electric charges of holes stored in the base and C is a capacitance connected to the base. As will be obvious from this expression, high integration results in reduced cell size, Q and C, so that the potential V.sub.p produced by optical excitation is maintained substantially constant. Therefore, the proposed system could be advantageous for achieving high resolution in the future.
However, in this conventional apparatus, the refresh operation to nullify the carriers stored in the base relies on the forward current flowing between the emitter and base. Thus, due to the use of a short-time refresh pulse the base potential after the refresh operation depends on the base potential before the refresh operation, which is a possible cause of a problem of afterglow and a nonlinear photoelectric conversion characteristic.
It is desirable that since the capacitor electrode 7 acts as a light screening area, the area of the capacitor electrode which receives light should be reduced as much as possible to thereby improve a proportion of an opening.